The present invention relates generally to energy-absorbing span lock system for drawbridges, and more particularly to a novel and improved apparatus for adjusting a pre-load in an energy-absorbing span lock system.
Span lock systems for drawbridges are designed to lock the outer ends of the two relatively movable leaves together when in the closed positions. Typically, a motor pushes a retractable steel locking bar longitudinally between wear shoes in a guide assembly on the outer end of one leaf into wear shoes in a socket assembly on the other leaf end. The locking bar maintains uniform deflection of the leaf ends by acting as a cantilever transmitting shearing stresses from one leaf to the other if there is any differential vertical loading due to vehicular traffic.
These systems generally allow a vertical clearance in the range of 0.010-0.025 inch between the sliding surfaces of the locking bar and the guide and socket assemblies under no bridge loads. However, wear due to vibration, pounding, poor maintenance, etc. the clearance may increase to 0.500 inch or more. This situation is dangerous and often manifests itself as problems or failures at other locations in the bridge such as in operating machinery gears and bearings, trunnion bearings, or live load shoe difficulties. My U.S. Pat. No. 5,327,605 issued Jul. 12, 1994 for an improved energy-absorbing span lock system for drawbridges is addressed to these problems. The wear shoes are continuously compressed against the top and bottom surfaces of the locking bar by energy-absorbing Belleville springs which cushion the effects of shock loads and vibrations. To prevent the locking bar from slamming against the shoes, the shoes are pre-loaded for zero clearance between the wear surfaces of the shoes and the locking bar by compressing the springs, under bridge no-load operating conditions, to maintain the shoes and locking bar in firm continuous contact. The springs have load-deflection characteristics which provide an approximate initial compression or pre-load when the shoes have been deflected by the fully inserted locking bar. The adjustment is usually done before the guide and socket assemblies are mounted on the bridge with out the locking bar inserted. Shims, composed of several laminations each 0.001 inch thick, are incrementally added as necessary in the vicinity of the springs until the deflection of the shoes, when the locking bar is inserted under no bridge load, compresses the springs to the desired pre-load. Adjustment is cumbersome due to the size and weight of the parts being handled, as well as time-consuming since it is a trial-and-error process that results in many assemble and disassemble iterations to add or subtract shims to reach the precise pre-load deflection. There is no provision for making these adjustments while in the assembled housing, and once installed on a bridge, any pre-load adjustment of the springs required to compensate for shoe wear, fatigue or deterioration of components is an extremely costly undertaking. The bridge must be taken out of service while the entire span lock system is removed to add or subtract shims.
Accordingly, it is an object of the present invention to provide a novel and improved drawbridge span lock system having an energy-absorbing adjuster device which can be pre-loaded a specified amount without clearance between the locking bar and the guide and socket assemblies under no-load conditions.
Another object of the invention is to provide an energy-absorbing device for a drawbridge span lock system in which a pre-load can be adjusted after being installed in bridge structure without prolonged interruption of service to vehicular traffic.
Another object of the invention is to provide an energy-absorbing adjuster device in a span lock system which can be precisely pre-loaded without clearance between the locking bar and the wear shoes to any of an infinite number of levels .
Still another object of the invention is to provide a method of pre-loading an energy-absorbing span lock system with substantially zero clearance between the locking bar and the guide and socket assemblies.
A still further object of the invention is to provide an energy absorbing device for a drawbridge span lock system which is easy to manufacture, operate and maintain.
These and other objects and of the invention are accomplished in a drawbridge span lock system in which a locking bar longitudinally slides between wear shoes in each of the guide and socket assemblies. The wear shoes are pre-loaded a selected amount by springs within the assemblies to maintain continuous contact of the shoes with the locking bar under no-load operating conditions of the bridge. Threaded adjusting carriers externally accessible in each assembly enable the springs to precisely pre-load the wear shoes to any amount within a design range.